Kozlov needed all his variety and savvy to keep Nava at bay, having no previous experience with Nava, who trains in Spain.

"Not let him hit big every ball," was Kozlov's answer to the question of how to play someone like Nava, who hits out on every ball. "Try to make him hit bad a little bit, throw in a couple of slices. I think I served really well today, and he couldn't return anything. I had to keep him thinking a lot, because if I just gave him forehands up the middle, he's going to blow me off the court. We didn't have that many long rallies to be honest, because he would miss, or he would hit a winner. I was just trying to be a wall and get everything back."

Kozlov beat Fritz last year in the 18s quarterfinals, but lost to him in the quarterfinals of a California Futures in January 6-4, 6-4.

"It should be a good one," said Kozlov. He's obviously the World No. 1 junior, he's playing well, he's got a lot weapons. I'm kind of the underdog here. At the Los Angeles Futures and he played a great match. That was one of his best matches, he told me after the match he played really well. We're pretty good friends, so it should be a good one."

Fritz and Mmoh, who is currently the No. 3 junior player in the world, had never played before either, making their match a highly anticipated one. Fritz broke in the second game of the match and never trailed, playing first-strike tennis on the quick hard courts of Stowe Stadium.

"I played well, I played a very good match," said Fritz, who injured his knee in his first match last Saturday, but has been able to play through any discomfort. "It [the knee] was off and on, some shots it hurt on in the first set, but in the second set, it felt great. I wasn't bending low on my serve or anything like that, but it's definitely getting better every day. Yesterday, I actually saw an improvement in it, and I was doing lunges on it, thinking oh my gosh, this is great."

In the second set, Fritz had to work hard for his first hold, giving Mmoh hope that he could advance to the semifinals for the second straight year. But Fritz broke in the fourth game, and played his aggressive baseline game, while also coming forward to end points.

"Mmoh's so fast that I have to come in to finish some points," said Fritz, who credited his recent three-week stint with the San Diego Aviators of World Team Tennis with improving several aspects of his game. "With Mmoh you've got to come in, because he's going to get the extra shots back."

Mmoh fought off three match points serving at 2-5, and two more with Fritz serving for it, after a forehand winner, a backhand winner and an ace gave him three match points. Fritz missed a forehand on the fourth match point and Mmoh outlasted Fritz in a long rally on the fifth, but the sixth match point ended it, with Fritz going behind Mmoh for a forehand winner.

Fritz doesn't expect the previous two meetings to have much impact on Saturday's semifinal.

"Last year I rolled my ankle and I just should have defaulted," Fritz said of his quarterfinal loss last year. "I was just limping around the court."

As for the January Futures win, "he definitely didn't play his best," Fritz said. "It was kind of a weird match, because I played the exact opposite of my game. I really pushed the ball the whole time. We were pushing backhands crosscourt the whole match. I don't know how this one is going to go. I think it's definitely going to be very close."

The 16s division produced the only three-set match of the day, with No. 4 seed Alexandre Rotsaert coming from a set and a break down to defeat No. 5 seed Sean Sculley 2-6, 6-4, 6-2.

Rotsaert, who said reaching the 18s Clay Court final last month gave him confidence coming into the tournament, was down a break at 4-3 in the second set and 2-1 in the third set, but each time he immediately broke back.

"I had to stay positive," said the 15-year-old, who lived in England as a child, before returning to the US, with his most recent residence in Boca Raton, Florida. "I was a little nervous at the start, making a lot of unforced errors. The courts are a little fast and I wasn't used to that. I had that problem in the last match I played in the round of 16, but I told myself I was going to fight. I thought of my game plan, going back to basics, one point at a time, and slowly, slowly I started playing a little better and by the end I was actually playing some pretty good tennis."

Sculley's tennis went in the opposite direction, with his forehand, usually so reliable in putting away short balls, began to go off. Whether due to fatigue or Rotsaert's improved play, Sculley's errors began to pile up and he lost the final five games of the match.

Rotsaert's performance this year has made up for a disappointing Kalamazoo debut in 2014.

"Last year, I came here, beat a seed, and I had to withdraw because of my back," Rotsaert said. "It was really a heart-breaker for me, so I'm really, really happy to be here. It's awesome. It's such a prestigious event."

Rotsaert suffered his back injury last year when he was playing his semifinal opponent this year, No. 8 seed Kyrylo Tsygura, who advanced with a 6-0, 6-3 win over No. 16 seed Jason Lui Friday morning.

Tsygura won the first eight games of the match, but Lui got it back to 2-2 in the second set, which provided a few minutes of concern for Tsygura, a 16-year-old from Maryland.

"In the second set, when he made it 2-all, I kind of felt a little bit of pressure, and I faced a break point at 2-all," said Tsygura, whose nickname is K-Money. "But I just stuck to my aggressive game plan the whole match, so I just came in an hit a volley winner. This is definitely one of the best matches I've ever played played in my life."

Tsygura believes his variety is a key factor in his success.

"I don't give two of the same balls to my opponent," Tsygura said. "Max two balls of the same. I just change it up and they always have to hit different shots and it can eventually frustrate them and they'll start going for too much. I've always been pretty crafty, and I'd say it's the best part of my game, that I can do different shots."

At last month's Team USA playoffs on clay in Boca Raton, Rotsaert defeated Tsygura 6-2, 6-1, and he is confident going into Saturday's match.

"I think it's all up to me," Rotsaert said. "I think if I play my brand of tennis, trying to come in, try aggressive and not let the other person dictate, I think I have a very good shot."

The doubles finals are set for Saturday. In the 16s, No. 7 seeds William Howells and Danny Thomas will play No. 8 seeds Bryce Pereira and Ivan Thamma. Howells and Thomas defeated unseeded Cody Lin and Andrew Ton 6-3, 6-4 in Friday's semifinals and Pereira and Thamma came back to eliminate No. 14 seed Jackson Allen and Carson Haskins 3-6, 6-2, 6-2.

Saturday will begin with both boys 16s semifinals (Patrick Kypson and JJ Wolf advance to the semifinals with wins on Thursday) at 9:30 am, followed by both 18s semifinals as the 16s finish. Frances Tiafoe and Tommy Paul reached the semifinals with wins on Thursday.

The 16s doubles final will begin at 1:30 pm, followed by the 18s doubles final.

Medical Update from the Nats at Kzoo…the Backdraw Flu continues to spread… all winners in the back draw quarters advance due to the "B.D.Flu" ..Mmoh ( wd.inj), Nava (No Show) Rubakov ( wd.inj) and Opelka ( wd.inj), …players..a reminder to get your B.D.Flu shots before entering the tournament..:)

It is EMBARRASSING that the players USTA is coaching at Kalamazoo is allowed to default the back draw at Kalamazoo.

They are going around the country to Regional Training Centers telling them how to coach and setting the standard but Jay Berger is saying that true "Development" is to default the back draw matches at a tournament.

This is a slap in the face to the tournament and setting a really bad example to all the players!

USTA Player Development has it backwards, The Private Sector should be telling the USTA how to coach because they are doing it 10 times better and has proven results. The USTA Player Development has zero results, only players they took from the private sector.

You make a great point about the message the USTA is sending about not playing matches for "development" because most commentators and even Jose Higueras has called American junior players soft.

The USTA PD are making their juniors entitled and soft. Why can't they finish the tournament? They do not need the matches? Last time I checked NONE of those who withdrew had great ATP rankings. Early in Rafa's career, he won a ATP Tour Event, then competed in a Challenger the following week, he didn't withdrawal. It's time for the USTA to walk-the-walk, instead of preaching what is only convenient to them. They need to lead by example not by having a monopoly based on US Open money and forced power which only keeps them relevant.

That goes along with my feeling that the USTA should not give wildcards to players who haven't competed in a number of USTA events during the year. Their development is one thing, but taking away the opportunity for others to develop by playing against the best players is another.

I think it is fair expectation that you should be prepared to play a tournament to its completion once you decide to enter it. This tournament has a backdraw; you don't like it, don't sign up. A legitimate injury is a different story but many of those defaulting from the back draw for 'injury' are clearly not hurt. My favorite is the 'no show' where they don't even the common courtesy for their fellow competitors to give them a heads up that they are quitting the tournament. I saw a post from George Opelka on Twitter where he said that these are very complicated decisions that are impossible to describe in 140 characters. While Reilly defaulted the back draw (while still in doubles no less), I respect George's opinion and would be very interested to hear him expound on their rationale.

I dont know the Opelkas but I completely understand his decision. When you have a match that can potentially earn you a trip to the main draw of the U.S. Open, don't you want to be rested and ready to go. Why would you play two singles matches earlier in the day prior to the much more important doubles match?

the USTA needs to be held accountable. A. Professionals should not be playing in this tournament, robbing spots from players that can be seen by college coaches.B. Defaulting on back draw. Then playing in doubles. Really. Is this where our USTA money is going? Totally gross

Colette, have you seen the withdrawels/no shows in the back draw of the national boy's 18's?If you really want to understand why American players are so fed up with the USTA and their entitled PD, this pretty much sums it up.

1) Us, the 99.9% want the match play and their players can't play? You sign up for a tournament, you finish it.2) US, the 99.9% have been described as "soft", yet their players who got wildcards can't play?

So, the parents/players pay their USTA tournament fees/hotel/airline/car rental./ and their own coaches and these players don't get their matches............. because the USTA PD players who pay no tournament fee ( wild card), no hotel, no airline, no car rental and no coach fee ( AS THE USTA PD PAYS FOR ALL OF THIS ) can't be bothored to play the back draw? Wow.

I will be writing a post devoted entirely to the issues raised by the back draw later this week. I think the discussion is valuable, but there is definitely some misinformation circulating right now in some of these comments.

I look forward to your discussion, Colette. I think it's absolutely wrong that players who default in the back draw due to "injury" are allowed to continue in doubles. (My tennis kid is in college and never played nationals so I don't have any personal dog in this fight either.) Surely that practice should be stopped.

It's disappointing that things haven't changed since my son played the Zoo, In 2012 Dennis Novikov was given wild cards by the USTA into both singles and doubles so he could play for the wild cards for the Open. Dennis won both singles and doubles while our boys, who had played the appropriate tournaments to qualify for Kalamazoo, were runners up in doubles and did not get to play in the main draw of the US Open. However, I remember a doctor at the Zoo who players needed to see before they could withdraw from the tournament. Is this not the case? Or, is that, too, a shame?

Anybody who has played a 2 or more matches in a week is going to get banged up and probably have at least something minor that could pass for an injury. And I get it, most of the top players are only playing for the USO WC, and don't care about ranking points earned in backdraw matches. The point is accountability, having respect for the tournament and its other players, and the game of tennis as a whole, having a sense of competitive pride and a desire to compete, and just plain old toughness and resiliency, mental and physical, after a loss or if your body is sore. These are all things that should be encouraged by parents and coaches, and unless a kid is physically unable to step on the court, or has a greatly increased risk of injury from playing, he should tough it out.

And I do worry that because so many of the top kids withdraw, it becomes the cool thing to do, and there will be a trickle down effect at all levels.

Certainly, I think a player should play every match at an event. And wildcards should put extra effort to play every match and prove their worth of a wildcard. However, I do think that a player/family may have to weigh a backdraw match against other factors including health, economics, work and school. So I think folks should get a 'pass' for the occasional and very rare BD flu, esp those on their own dime. But even San Diego had a string of backdraw illinesses from players that were still around the next day. It's really hard to understand defaulting out of a 3rd / 4th place match in both National Championships. Although I understand the logic of "save it for doubles with the US Open ticket on the line".... The occasional and rare backdraw withdraw isn't horrible, but there are players who NEVER play backdraw matches and I have a harder time with that. And of course, you just hate to see it at the National Championships. Finally, the USTA PD players play the USTA events so rarely, I agree that it's a crime for them not to play every match and make themselves available to compete against all comers. Entitlement is the biggest thing holding US players back, IMHO.

Tennis is an individual sport. Junior players are not part of a team in these events, they do what is best for them, individually. They do not owe lower players a thing, even setting an example. When the lower players get to their level, they can choose to withdraw as well, rather than risk taking any kind of chance to get injured when there is no reason to.

Tennis is simply a selfish sport, just go to an adult league match, do those people talk about anything but themselves?

Entitlement is becoming a huge problem in all aspects of daily life in the US. Probably in Western Europe as well by now, I am guessing. Mothers bring their children to day care even on the days they don't work, because they need "me time." Such idea didn't exist not too long ago and not just for mothers, or kids or any member of the society. In tennis, one example that I read recently about is Denis Kudla, who left USTA and many of the financial benefits they provided and said something along the lines that now that he is paying the coach and everything else so much more is at stake. I think his results show huge improvement. My point is, no matter how great junior results these kids have, playing back draw or not is not that important, what is important is that they feel like superstars and act like it, many of them, yet they truly have no reason to. In terms of life success and pro tennis success,they are like Kardashians,(not all, we know well that a couple of these juniors have had amazing pro success for their age) full of themselves yet without any valid reason.